A membrane is a thin structure which is permeable to some substances or phases and which is impermeable for other substances or phases. A membrane can be used for material separation. In membrane distillation, a thin, microporous and hydrophobic structure is used as a membrane wall which is permeable to gases, but impermeable to liquids, particularly to water. The thin microporous and hydrophobic structure may be applied to a support layer to enhance the mechanical strength of the membrane wall. In membrane distillation, a liquid flows along the membrane wall, wherein the low-boiling components of the liquid evaporate via the membrane wall due to a differential partial pressure. The membrane wall is used for phase separation and allows a very large area of the phase boundary for the evaporation to be generated. The evaporated ingredients are condensed on the opposite side after passing through the membrane wall and can thus be recovered in liquid form and withdrawn from the process.
Within the membrane pores usually gas is present during the membrane distillation. However, it may occur that the membrane wall is wetted and that liquid enters the pores of the membrane wall. In this case, the liquid retention by the membrane wall is no longer ensures and liquid can pass through the membrane wall.
A wetting of the membrane wall can be effected by crystallization of salts on the surface of the membrane wall, by deposition or by surface-active substances which come in direct contact with the membrane wall.
A membrane wetted by a liquid can be replaced individually or together with the entire distillation apparatus. However, this involves considerable effort and is also an uneconomical solution.